Hycean world Archives

Planets that will be observed with MIRI in transit, in JWST Cycles 1-5. As mentioned in section 5, we see a wide variety of planet sizes and equilibrium temperatures. The currently published planets are shown in black dots, and listed in Table 1 and Figure 1. We have ommitted the white dwarf planet WD 1856 b (Teq = 160K, Rp = 10.2R⊕, cycle 4), as well as the disintergrating planets K2-22 b (see section 2.1) and BD+05 4868 A b (Teq = 1820K, unknown Rp, cycle 4). -- [astro-ph.EP]

A Homogeneous Survey Of JWST MIRI Transmission Spectra Of 10 Exoplanets

by Keith CowingSeptember 7, 2026September 7, 2026

[astro-ph.EP] The launch of JWST has enabled unprecedented atmospheric characterization of exoplanets. In particular, the JWST-MIRI instrument in the mid-infrared enables probing of molecular species that are harder to detect […]

Geometry of the LTT 3780 planetary system at the time of our radio technosignature observations. The left panel shows the sky-plane (X–Y) view as seen from Earth, while the right panel shows the top-down (X–Z) view of the system. The orbits of planets b (blue) and c (green) are shown relative to the host star (yellow). Our observations from the ATA, mentioned in Table 1 on 5th November 2025, were timed to coincide with a planet–planet occultation (PPO), when planet b passes behind planet c along the line of sight, providing a unique configuration to search for radio technosignatures during mutual planetary alignment.-- [astro-ph.IM]

SETI & Technosignatures

A Search For Narrowband Technosignatures From LTT 3780 With The Allen Telescope Array And The Karl G. Jansky Very Large Array

by Keith CowingJuly 9, 2026July 9, 2026

[astro-ph.IM] The LTT3780 system hosts two known exoplanets-LTT3780b, a rocky super-Earth, and LTT3780c, a temperate sub-Neptune-orbiting a nearby M dwarf on opposite sides of the radius valley. LTT3780c has been […]

Diagram of the dominant chemical reaction pathways in our photochemical network.

Atmospheres, Climate, Weather, Biochemistry & Organic Chemistry, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Imaging & Spectroscopy, Press Release, Water/Hycean Worlds & Oceanography

A Hycean Interpretation of K2-18b Supported by Photochemical Atmospheric Compositional

by Keith CowingMay 20, 2026May 20, 2026

The nature of the sub-Neptune K2-18b is debated between Hycean and mini-Neptune interpretations.

Summary of our method and comparison to the traditional grid modeling approach. -- astro-ph.EP

Astrogeology, Atmospheres, Climate, Weather, Exoplanets, -moons, -comets, Status Report, Water/Hycean Worlds & Oceanography

Unusually Hot Interiors Could Reconcile the Missing Methane Problem for Warm-to-Hot Exoplanets with Hydrogen Atmospheres

by Keith CowingApril 4, 2026April 4, 2026

JWST is revolutionizing the field of exoplanet atmospheres by delivering unprecedented spectroscopic constraints on their chemical compositions. It has provided tight constraints on the abundances of dominant carbon- and oxygen-bearing […]

A schematic diagram of the hycean Lotka-Volterra model we implement in this work. The schematic shows how various parameters in our model are connected in a vertical, one-dimensional water column.

AI - Data - Apps - Cybernetics, Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Gaia - Planetary Perspectives, Habitable Zones, Proxima Centauri, Status Report, TRAPPIST-1, Water/Hycean Worlds & Oceanography

Ecological Modelling Of Hycean Worlds

by Keith CowingMarch 25, 2026

New observations are opening the possibility of characterising habitable environments in exoplanetary systems, with the recent example of the candidate hycean world K2-18 b.

JWST transmission spectrum of TOI-270 d. Orange errorbars correspond to NIRISS observations, presented in this work, while dark red errorbars correspond to NIRSpec G395H observations presented by HM24. The NIRISS observations were binned down to R=25 for order 1 and R=12 for Order 2 for visual clarity. The blue curve denotes the median retrieved spectral fit obtained with our canonical retrieval, described in Section 3. Darker and lighter turquoise shaded regions denote the spectral 1- and 2-σ contours corresponding to the spectral fit. Yellow dots denote the median spectrum binned to the resolution of the datapoints. The NIRISS data are vertically offset by +28 ppm, which is the median offset retrieved for our canonical model. -- astro-ph.EP

Astronomy & Telescopes, Atmospheres, Climate, Weather, Exploration Gear & Tech, Imaging & Spectroscopy, Status Report, Water/Hycean Worlds & Oceanography

The Atmospheric Composition Of TOI-270 d

by Keith CowingNovember 22, 2025November 22, 2025

The first explorations of temperate sub-Neptune exoplanets have been the hallmark of early JWST observations.

Wind fields in our imposed TOA 𝐴𝑏 = 0.6 case.

Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Exoplanets, -moons, -comets, Press Release, Water/Hycean Worlds & Oceanography

General Circulation Models Of Hycean Worlds

by Keith CowingNovember 12, 2025November 12, 2025

Sub-Neptunes represent the current frontier of exoplanet atmospheric characterisation. A proposed subset, Hycean planets, would have liquid water oceans and be potentially habitable, but there are many unanswered questions about […]

Regime diagram showing four distinct climate regimes, according to our semi-analytical model. The solid blue line represents the instellation above which an atmosphere enters an inhibited state, dashed lines represent the instellation below which an atmosphere becomes fully convective (green versus brown assumes the atmosphere above the inhibition layer follows a moist versus dry adiabat). Arrows indicate the direction of transition. In the lower left of parameter space, atmospheres are convective (blue), in the upper right they are inhibited (red); note, atmospheres in the inhibited regime additionally show multistability (see Section 4). Between the convective and the inhibited regimes, thicker atmospheres are bistable (light and dark yellow) and thinner atmospheres oscillate (light and dark green). All parameters used in the semi-analytical model are listed in Table 1. -- astro-ph.EP

Atmospheres, Climate, Weather, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Habitable Zones, Press Release, Water/Hycean Worlds & Oceanography

Bistability, Oscillations, And Multistability On Hycean Planets

by Keith CowingOctober 29, 2025October 29, 2025

Hycean planets are hypothetical exoplanets characterized by H2O oceans and H2-rich atmospheres.

A classification scheme for volatile-rich sub-Neptunes. The schematic compares different types of sub-Neptunes with H2 and H2O dominated atmospheres as a function of the equilibrium temperature and H2 content. Ice worlds: Cold subNeptunes where any water is frozen in a layer of ice atop high-pressure ice, with a thin H2-dominated atmosphere (65). Hycean worlds: Warmer than ice worlds, hycean worlds have a thin H2-rich atmosphere overlying an ocean of liquid water, which itself is above a layer of high-pressure ices. These planets have the potential to support habitable conditions (9). Mini-Neptunes: Sub-Neptunes with a deep H2-rich atmosphere followed by a mixed H2O-H2 supercritical layer below before reaching a layer of high-pressure ice and then a core (17, 65). Steam worlds: Water worlds with H2O dominated atmospheres. At equilibrium temperatures above Tcrit, a high proportion of H2O exists in the atmosphere, with a supercritical mixed-envelope at higher pressures. Supercritical mini-Neptunes: These planets are hotter than Tcrit, and contain interiors where H2 gas is highly soluble in supercritical H2O (46, 65). This mixed-envelope would lie over a layer of high-pressure ice above the core. Legend: The legend at the top shows the different compositions corresponding to the colors in the schematic, including the core, different phases of H2O and envelope composition; high-P ice refers to high-pressure ice. Note that the core has further distinct layers but we do not label them here. Sharp phase transitions are shown for ice-atmosphere and ocean-atmosphere transitions. Other phase transitions are marked by a gradient, as they may not be sharp phase transitions (83). -- astro-ph.EP

Astronomy & Telescopes, Exoplanets, -moons, -comets, Imaging & Spectroscopy, Status Report

Exploring The Sub-Neptune Frontier With JWST

by Keith CowingSeptember 25, 2025September 25, 2025

Sub-Neptune planets, with sizes and masses between those of Earth and Neptune, dominate the exoplanet population.

Atmospheres, Climate, Weather, Biogeochemical Cycles & Geobiology, Biosignatures & Paleobiology, Exoplanets, -moons, -comets, Gaia - Planetary Perspectives, Habitable Zones, Press Release

Exoplanets Are Not Water Worlds

by Keith CowingSeptember 19, 2025

An exoplanet orbiting a dwarf star 124 light-years from Earth made headlines around the world in April 2025. Researchers at the University of Cambridge reported that planet K2-18b could be […]